[llvm] [BOLT] Add pre-parsed perf script support (PR #163785)

[Ádám Kallai via llvm-commits]

kaadam wrote:

@aaupov Thanks for the review.
I've also added a unit test for parsing the perfscript header.
Another test plan: I plan to add a `perf-test` that covers both 'perfscript' generation and the parse. What do you think,  is that good for you to do in a separate PR?

https://github.com/llvm/llvm-project/pull/163785